The terminals of such a network transmit and receive data in the form of cells of fixed length. While cells are being transferred through the various nodes constituting the network, each cell is stored for a greater or lesser length of time in queues, and cells are lost when certain queues overflow. In order to avoid slowing down network operation, no provision has been made until now in the network itself for acknowledging reception of cells or for detecting the loss of cells. Because network management is so simple, very high data rates are possible, and these may reach 600 Mbits per second. Quality of service and the efficiency with which network resources are utilized vary as a function of resource requests, corresponding to various connections being established that share a set of resources in common.
Until now, this efficiency and this quality of service have been accepted as being adequate without it being necessary to provide traffic flow management within the network, because the vast majority of terminals taken into consideration up to the present deliver data at fixed rates. However, more and more applications, such as high definition television, are now being envisaged for which there could be very many terminals operating at data rates that are high and variable. If the majority of the data transmitted by an asynchronous time multiplexed network is at a variable data rate, it becomes necessary to consider regulating traffic flows in order to optimize resource utilization and quality of service.
Asynchronous transfer mode switching networks constitute merely a special case of packet switching networks. Prior art packet switching networks provide a highly specialized service, namely consulting data bases. Traffic flow through each connection is managed by making use of the flexibility in the data sources and receivers. This flexibility is achieved by storing data in buffer memories, with such storage being limited within tolerances defined by the transfer protocol used by the network.
A prior art protocol called X25 does not impose any particular method of managing traffic flows in a network, but when used at each node in the network it makes it possible to control the traffic flow over each connection. Each node is in full control of the data flow rate it receives from adjacent nodes. The network also controls data sources indirectly: each node is capable of regulating adjacent nodes and the regulation effect can thus propagate to the data source. Such a network includes a service controller at each inlet/outlet port in each node. This structure suffers from the drawback of limiting the data rates of connections since interchanges of service information between nodes take up time.
In so-called "frame relay" prior art networks, terminals include service controllers that apply a protocol called LAP-D There are no service controllers applying this protocol within the nodes of the network. Unlike networks using the X25 protocol, the nodes of the network cannot explicitly accept or reject frames. Because of this, the method of managing traffic flow uses certain functions of the service controllers in the terminals and applying the LAP-D protocol. The functions made use of are the following:
transmitting an acknowledgement whenever the terminal in question receives a call;
continuing to transmit cells after having transmitted a first cell and while waiting to receive an acknowledgement corresponding to the first cell, and limiting the number of cells transmitted while waiting to a predetermined number;
then suspending cell transmission during a predetermined duration if the acknowledgement is not received before the predetermined number of cells have been transmitted;
then retransmitting the first cell if the corresponding acknowledgement is not received during the predetermined duration;
and then releasing the connection occupied by the terminal under consideration if the acknowledgement is not received after a predetermined delay measured from the first transmission of the first cell.
The network can regulate the data traffic flow over a connection by delaying data frames or by eliminating them. These actions are tolerated by the terminal transmitting the data and by the terminal receiving the data. Their effect is to cause frames to be repeated and to give rise to periods during which no frames are transmitted, thereby having the overall effect of reducing the data rate and thus of reducing network congestion.
These known methods of managing traffic flows rely on tolerances defined by the values for the parameters as predetermined for the protocols X25 and LAP-D, respectively. These parameter values are determined as a function of data delays and data losses that can be tolerated on connections that are specialized in consulting data bases. However, depending on the type of service under consideration, the consequences of a delay or of a loss of data are not at all the same. Data base consultation does not tolerate any loss of data, but it tolerates data delay very well.
A telephone link cannot accommodate a delay of more than 100 ms, however it can accept the loss of a few packets of data without these packets being retransmitted. In constrast, a link transmitting television pictures can accept a delay of several seconds providing the delay between two predetermined terminal points is limited, with the terminals being provided with memory suitable for smoothing variations in delay. However the pictures transmitted may be very sensitive to data loss, particularly if the encoding method is a differential method that makes use of earlier pictures for decoding the current picture. Prior methods of managing traffic flows in a network are thus not directly usable in a wideband integrated services network using asynchronous transfers.
An object of the invention is to provide a method of managing traffic flows in a wideband asynchronous time multiplexed network integrating various types of service, with the method taking account of the type of service on each connection for the purpose of simultaneously optimizing resource utilization and quality of service, and with the method not requiring the use of service controllers at each node of the network.